The following are the sections that we will be covering on propeller work in a dry dock:
The major components of a propeller are:
Propellers should be maintained in top condition at all times. The main factors that detract from optimum condition are fouling, cavitations and physical damage. Any distortion from their true shape can cause an imbalance and hence vibration, which in turn causes increased cavitations, loss of thrust, drive shaft damage, wear on the numerous bearings, and increased fuel use due to decreased efficiency.
The simplest form of maintenance on a propeller is to clean it. Even a 1mm layer of accumulated fouling or calcium deposits on a propeller will significantly increase its roughness, and within 12 months or so can increase an ISO class I to an ISO class II, or a class II to a III. This causes large increases in fuel consumption. Practical figures and elaborate tests indicate a 6 to 12% gain in fuel consumption in polishing a propeller from a class III condition to a class I condition. Some propellers support marine growth up to 20 mm thick, which obviously has a major effect.
Cleaning, or polishing, involves the removal
of this growth, leaving behind a clean and highly polished surface. The
manner in which this is done is important. If the growth is removed rapidly
using harsh abrasives, the surface of the blades may look shiny, but they
will be deeply scratched. These scratches will themselves impart a roughness
that is beyond the 1.6 micron (CLA) Ra* tolerance required for a class
I propeller. Additionally, the scratches will provide an ideal key for
further calcification, micro- and macro-fouling, speeding up the fouling
process and often making things worse than they were before. The correct
procedure for polishing a blade leaves a very smooth surface, which will
resist future fouling. Although slightly more time consuming to achieve,
this level of finish is desirable as its long-term effects on fouling
and engine wear more than compensate for the initial costs.
When to polish?
A propeller polished to a 1m finish will
maintain it's efficiency for approximately 9 months, however, depending
on the type of vessel and it's trading patterns, this may be reduced.
For example, vessels with longer layover periods will incur a higher fouling
penalty, as the propeller is stationary. Vessels trading constantly will
have less of a fouling problem. It has been proven propellers polished
to a 1m finish still being close to optimum conditions after 10 months
Propellers can easily be damaged if they
strike a buoy, or hit floating debris or ice. The damage is usually to
the tip, which can become bent or which can even have large chunks of
metal taken out of it. Physical damage of this nature is what most commonly
causes vibrations. The solution in this case is to trim the blades equally
to remove the damage and achieve a proper balance, and reduce excessive
cavitations. The techniques used in this eventuality rely on extremely
accurate measurement of the diameters of the individual blades to reduce
to a minimum the amount of material removed, but also to ensure a perfect
balance. Bad trimming can result in an even worse problem, and cases are
on record where inaccurate trimming has resulted in propellers being damaged
beyond repair, or even destroyed completely.
There are 3 main types of modifications:
1) Diameter reduction - Easily and inexpensively performed under water, this is the usual method for increasing rpm and balancing the ratio. The blade tips are cropped and faired.
2) Pitch reduction - This involves twisting the blades and can only be accurately done in a workshop as the blades need to be heated to prevent cracking. The twisting usually distorts the hub, which requires machining to ensure a proper re-fitting to the shaft. Although more expensive, this is one of the most effective modifications as there is no loss of blade material. It is ideally suited to blades smaller than 4,000mm dia.
3) Trailing edge modification - This is achieved
by either bending the trailing edges, or by cutting them. Both operations
can be performed in-water and can achieve an effect on the rpm of approximately
5%. In bending, the trailing edge is slightly modified, without loss of
material or strength of the blade. With cutting, a small amount of trailing
edge material is removed and the edge re-ground. Templates are used to
ensure accuracy, and the effects can be calculated to within approximately
1% rpm accuracy.
The stern tubes, Plummer blocks, bulkhead glands, thrust blocks and gear cases shall be examined for cracks and distortion. The propellers & shaft sections shall be checked for bending. Shafts shall be disconnected as necessary to enable readings to be taken. The shaft alignment, alignment of gearing to shafting and of turbines to gearing, shall be checked. The main gearwheel rim and teeth and associated pinion teeth shall be checked for distortion.
In the event of severe damage to the final reduction gear, the remaining gear meshes and gearbox internal couplings shall be examined. Propeller shaft flexible output coupling, shall be examined for damage to flexing elements and bearing and sliding surfaces, and for distortion and cracking of main components. Where the main propulsion machinery is mounted on a raft, the alignment of the engines to the main gearing shall be checked.
The CO will provide the Docking Officer with the certificate in duplicate, this certificate will state that:
• All sea connections, underwater valves
and fittings are in efficient working order.
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